“The FBI said they were done by the hands of a genius – well, that’s me. How strange it feels!” So said Chinese painter Pei-Shen Qian to Bloomberg Businessweek in December 2013, about works he created in his garage in New York.

He is accused of being part of an $33m art swindle, and is still at large. The latest arrest in the case was made on Friday in Spain – on Monday a US grand jury indictment was unsealed, listing the methods the network is alleged to have used to fool collectors and experts. It is so detailed it has been described as a “forger's manual” by The New York Times.

Those prosecuting the conspirators are not the only ones to reveal inside tricks. In the past few months, master forger Wolfgang Beltracchi has given interviews to CBS, Channel 4 News and The Sunday Times – despite currently serving a six-year sentence for fraud.

The German painter – credited with carrying out the greatest fake art scam in history, creating hundreds of paintings in the style of famous artists including Henri Matisse, Georges Braque and Max Ernst – claims to channel the dead artists he is copying. Imprisoned in 2011, he is allowed out on day release to his home in Cologne, where he has revealed some of his insider tricks. Here are seven insights into the world of the top forgers.

Beltracchi and his wife Helene have been described as ‘the Bonnie and Clyde of the art world’. Together they went to extraordinary lengths to make the forgeries seem true, claiming Helene had inherited the collection from her grandfather, and that he had hidden it from the Nazis on his country estate in Germany before World War II. The couple even staged a mock-1930s photo in which Helene posed in costume as her grandmother, with Beltracchi’s forgeries on the wall behind her. The picture was taken with an old box camera and developed on pre-war photographic paper for extra verisimilitude.

2 The right materials

The Beltracchis made art gallery labels for their pieces, ageing them with tea and coffee, while the indictment in the current FBI case alleges the accused “stained newer canvases with tea bags to give them the false appearance of being older than they actually [are]”. They “also acquired old furniture at flea markets and other places to obtain masonite, a hardboard used in that furniture that was also used by the abstract expressionist artists whose style [they] mimicked”. Flea markets appear vital to the successful forger: both groups used them to source canvases from the right periods.

3 Careful choice of paint

The indictment alleges that Qian was provided with “old paint that was created in the era in which the Fake Works would have been created if they were authentic works”. The Beltracchis also sent paints to labs to check they were available at the time the artist had painted. But pigment was their comeuppance: a gallery ordered chemical checks on a painting attributed to the 20th-Century Dutch painter Heinrich Campendonk which revealed traces of titanium white, a pigment not in use when the artist was alive.

4 Ageing well

One of those accused in New York, Jose Carlos Bergantinos Diaz, is said to have “subjected many Fake Works to various processes, such as heating them, cooling them, and exposing them to the elements outdoors” – including holding a blow-dryer over one of the paintings to make it appear older. The Beltracchis sped up the ageing process in their own home-made oven.

Helene Beltracchi scoured flea markets for frames fitting the period of her husband’s forgeries: the couple took apart the frames and collected the dust inside to use on the finished work. In turn, among Qian’s belongings, the indictment lists “auction catalogues containing works by famous American abstract expressionist artists... and other materials, such as an envelope of old nails marked ‘Mark Rothko’, all to aid Qian in creating the Fake Works”.

6 Borrowed expertise

According to the indictment, the defendants researched the lives of figures in the art world, including collectors and brokers, to create a false chain of ownership. That included a personal friend of one of the alleged conspirators, a Spanish collector who was interviewed about his life to add credence to their claims. In turn, Beltracchi says his work was mostly sold in Britain, America and Japan through leading dealers and auctioneers. He told The Sunday Times: “Selling fake paintings is dead easy. If they get a certificate from experts, the dealers will not ask too many questions, while the experts, who are mostly incorruptible, simply fail to detect my fakes because they are too good.”

7 Being original

Beltracchi claimed never to have copied a painting, instead pinpointing gaps in an artist’s body of work and creating a missing painting that could have existed in an approach that has been called “method acting on canvas”. He absorbed the painter’s style completely: when painting a piece supposedly by lefthander Raoul Dufy, he used his left hand; when forging a piece by Max Ernst, who had done much of his work on a wooden floor, he painted on a wooden bridge outside his home. And people wanted to believe the myth: one of his Max Ernsts appeared in New York’s Metropolitan Museum of Art, while another sold for $7m – when its owner discovered it was a fake, he decided to keep it anyway, saying it was one of the best Ernsts he’d ever seen.

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